Hydronix Hydro-Probe XT User Manual page 44

Chapter 5
The equation of the calibration line is defined by the Gradient (B) and the Offset (C), see
Figure 34. These values are referred to as the calibration coefficients.
The Gradient (B) is the relationship between the change in Moisture % to changes in the
Unscaled value.
Gradient =
The Offset (C) is the Moisture % value that corresponds to 0 Unscaled units.
The sensor will output 0 Unscaled units when in air. When loaded with dry material the
Unscaled value will increase and the Moisture % value will rise to indicate 0% Moisture.
The SSD value is a third coefficient used to define absorbed water within a material, this is
covered in section 2.2.
Using the coefficients the conversion from Unscaled to Moisture % is as follows:
In rare cases when the measurement of the material exhibits non-linear characteristics, a
quadratic term can be used in the calibration equation as shown below.
Moisture % = A x (Unscaled value)² + B (Unscaled value) + C – SSD
Use of the quadratic coefficient (A) would only be necessary in complex applications and for
most materials the calibration line will be linear in which case 'A' is set to zero
SSD coefficient and SSD moisture content
2.2
Saturated Surface Dry (SSD) is an offset adjustment typically used in aggregates and hard
materials to define the % of moisture that is tightly bound to the material and not freely
available. The use of the SSD offset allows the % of free water to be measured.
For applications where Total Moisture is required the SSD Value is left to zero.
Bound Moisture
In order to determine the moisture content of a material it is weighed, dried and re-weighed. In
practice it is not possible to easily determine the point at which the material reaches its SSD
condition. It is therefore often only practical to obtain an oven dried sample (Total Moisture).
SSD values are derived through more lengthy industry standard tests or supplied by the
material supplier.
Oven dried
moisture % (Total)
44 Hydro-Probe XT User Guide HD0538 Rev: 1.6.0
Change in Moisture %
Change in Unscaled Value
Moisture % = B x (Unscaled Reading) + C – SSD
+ Free Moisture
Water Absorption Value %
-
(SSD offset in the sensor)
Sensor Integration and Calibration
= Total Moisture
surface moisture %
=
(free moisture)